U.S. Pat. No. 4,263,519 (Otto A. Schade, Jr.), assigned to the assignee of the present patent application, shows various bandgap voltage reference circuits. FIG. 5 of the U.S. Pat. No. 4,263,519 is reproduced herein and
denoted as FIG. 1. A bandgap voltage (E.sub.BG), which is relatively accurate, is generated between terminals 69 and 30. If resistor 61 is equal to resistor 62, then a reference voltage (E.sub.REF) which is equal to 2 E.sub.BG is generated at output terminal 70. The operation of the circuit of FIG. 1 is well known and its description in U.S. Pat. No. 4,263,519 is incorporated herein by reference. One limiting factor on the accuracy of the circuit of FIG. 1 is that base current is drawn from node 69 to drive transistors 31 and 32. This base current, even through it is typically only a small fraction of the current flow through resistor 61, limits the accuracy of the voltage generated at output terminal 70. Some applications require greater accuracy than can be achieved by the circuitry of U.S. Pat. No. 4,263,519.
U.S. patent application Ser. No. 07/220,712 (Otto A. Schade, Jr.), filed July 18, 1988, and assigned to the assignee of the present application, is directed to current compensation circuitry for use with a junction transistor bandgap voltage reference circuit. FIGS. 2, 3 and 4 herein are very similar to FIGS. 2, 3 and 4 of the above cited patent application. FIG. 2 shows a reference voltage generator circuit with current compensation 100 comprising a reference voltage generator 102 and current compensation circuitry 104. The reference numbers used for the components and terminals of circuitry 102 are the same as those used for the corresponding components and terminals of the circuitry shown in FIG. 1 with a "0" added there after. Reference voltage generator 102 is the same as the prior art reference voltage circuitry shown in FIG. 1 herein except for the addition of a bipolar n-p-n transistor 22 which has its base coupled to the output of the operational amplifier 330 and to a terminal 106, its emitter coupled to a terminal 700 and its collector coupled to a terminal 200. Transistor 22 acts as a buffer between operational amplifier 330 and resistor 610. One limitation of the accuracy of reference voltage generator 102 is that base current needed to bias n-p-n transistors 310 and 320 is drawn from resistor 610 via node (terminal) 690. Consequently, the current through resistor 610 is not identical to the current through resistor 620. Thus if resistor 610 is equal to resistor 620, the voltage generated at node 700 is not equal to twice the voltage generated at node 690. Furthermore, the base current for transistors 310 and 320 varies with the current gains (betas) of the transistors and with temperature. Current compensation circuitry 104 generates a compensating current which flows into node 690 and which is essentially identical to the base current which flows from node 690 into the bases of transistors 310 and 320. The base current normally drawn from node 690 by the bases of transistors 310 and 320 is replaced by current compensation circuitry 104 and thus essentially the same current flow through resistors 610 and 620. The accuracy of the output voltage E.sub.REFO appearing at terminal 700 relative to the bandgap voltage V.sub.BG appearing at node 690 of reference generator circuitry 102 is therefore improved, typically by an order of magnitude or better.
Current compensation circuitry 104 comprises a two input operational amplifier 112, a current mirror circuit 118, an n-p-n transistor 120 and a resistor 124. Transistor 120 and resistor 124 may be denoted as a load element or as a dummy load element. Node 690 is coupled to a positive input terminal of operational amplifier 112 and to a second (slave) output terminal of current mirror circuit 118. A negative input terminal of operational amplifier 112 is coupled to node 116 to which is connected a first (master) output terminal of current mirror circuit 118 and the base of transistor 120. An output terminal of operational amplifier 112 is coupled to node 114 to which is connected an input terminal (generally denoted in the art as a common terminal) of current mirror circuit 118. The emitter of transistor 120 is coupled to node 122 to which is coupled a first terminal of resistor 124. The collector of transistor 120 is coupled to a terminal 200 to which is applied a positive voltage+VO. A second terminal of resistor 124 is coupled to a terminal 300 to which is applied a reference voltage which is shown as ground.
The operational amplifier 112 and the current mirror circuit 118 cause the potential of node 116 to be essentially the same as the potential (V.sub.BG) of node 690. Transistor 120 is designed to be the equivalent of transistors 310 and 320 and resistor 124 is designed to be equal to the equivalent of resistors 340, 360 and 350. If the same power supplies and base voltages are applied to transistors 310, 320 and 120, then the same total current that flows into the bases of transistors 310 and 320 is equal to the current flowing into the base of transistor 120 (node 116). Thus the current flow from node 690 into the bases of transistors 310 and 320 is supplied into node 690 by current mirror 118. Accordingly, circuitry 104 supplies all of the base current for transistors 310 and 320 and thus all the current which flows through resistor 610 also flows through resistor 620. This improves the accuracy of the voltage E.sub.REFO appearing at the output terminal 700 of reference voltage generator 102 by typically an order of magnitude or better.
FlG. 3 shows current compensation circuitry 104 with circuitry of operational amplifier 112 shown within a dashed line rectangle 112a and circuitry of the current mirror circuit 118 shown within a dashed line rectangle 118a.
Operational amplifier 112 comprises Field Effect Transistors (FETs) 124, 126, 128 and 130, an n-p-n bipolar transistor 132 and a resistor 138. Current mirror circuit 118 comprises FETs 134 and 136. FETs 124 and 126 are both n-channel Metal-Oxide-Silicon (MOS) FETs and FETs 128, 130, 134 and 136 are all p-channel MOS FETs. The gate of transistor 124 is coupled to the drain of FET 136 and to node 690. The sources of transistors 124 and 126 are coupled to a first terminal of resistor 138 and to a node 144. Second terminals of resistors 138 and 124 are coupled to terminal 300 and to ground potential. The sources of transistors 128 and 130 and the collectors of transistors 120 and 132 are coupled together to terminal 200 and to. positive voltage +VO. The drain of transistor 124 is coupled to the gates of transistors 128 and 130, to the drain of transistor 128 and to a node 140. The drain of transistor 126 is coupled to the drain of transistor 130, to the base of transistor 132 and to a node 142. The emitter of transistor 132 is coupled to the sources of transistors 134 and 136 and to node 114. The gates of transistors 126, 134 and 136 are coupled to the drain of transistor 134, to the base of transistor 120 and to node 116. The emitter of transistor 120 is coupled to one terminal of resistor 124 and to a node 122.
Transistors 134 and 136 serve as the master and slave legs, respectively, of the current mirror 118. The current that flows through transistor 134 is duplicated and flows through transistor 136. Thus the current that flows into the base of transistor 120 is essentially the same as flows into node 690 from transistor 136. The gates of transistors 124 and 126 draw essentially no current out of nodes 690 and 116, respectively, since the input impedances of transistors 124 and 126 is high as they are both FETs. Transistor 120 and resistor 124 are the equivalent of transistors 310 and 320 and resistors 340, 360 and 350. Furthermore, the supply voltages,+VO and ground, used for power are identical. Hence, the current flowing into the base of transistor 120 is essentially equal to the sum of the currents flowing into the bases of transistors 310 and 320. In view of this it is clear that the current needed to bias transistors 310 and 320 is supplied by compensation circuitry 104. Thus the current which flows through resistor 610 is the same as flows through resistor 620 and accordingly the accuracy of voltage generator circuitry 102 is improved.
However, operational amplifier 112 increases the complexity and the manufacturing cost of compensation circuitry 104. In many applications it is thus desirable to have circuitry which provides compensating base drive for a bandgap voltage reference circuit, but which uses fewer components and is simple to manufacture.